In the prior art, for example, there is known a drive unit for a hybrid vehicle, in which power of an internal combustion engine is distributed to an output member and to a motor generator through a planetary gear mechanism, in which the motor generator is used as a reaction member, and in which a revolution frequency of the engine is set to achieve a preferable fuel economy in accordance with a revolution frequency of the motor generator. According to this kind of drive unit, an engine revolution and an output torque can be controlled by the motor generator, therefore, the planetary gear mechanism and the motor generator function substantially as a transmission. Here, since the motor generator functions as a generator and establishes reaction force when the engine is outputting power, an electric power generated by the motor generator is supplied to an electric motor connected with the output member, and the power outputted from the electric motor is applied to the output member.
However, according to the drive unit thus configured, a large capacity electric motor applying power to the output member is required in order to obtain large drive force while operating the engine under the condition where the fuel economy is optimum. For this purpose, a drive unit for a hybrid vehicle disclosed in Japanese Patent Laid-Open No. 2003-127681 is adapted to suppress an increase in size of an electric motor, and to obtain driving force in accordance with a various vehicle speeds. Specifically, according to the invention disclosed in Japanese Patent Laid-Open No. 2003-127681, an internal combustion engine is connected with a carrier of a planetary gear mechanism, a first electric motor/generator is connected with a sun gear of the planetary gear mechanism, and a member of an input side of a transmission is connected with a ring gear. A member of an output side of the transmission is connected with a propeller shaft, and a second electric motor/generator is connected with the propeller shaft.
According to the drive unit disclosed in Japanese Patent Laid-Open No. 2003-127681, gear stage regions of the transmission are set in accordance with a vehicle speed, a gear stage corresponding to the gear stage region to which the running condition belong is set by the transmission, and the engine revolution is controlled by the first electric motor/generator to optimize the fuel economy. For this purpose, the second electric motor/generator is arranged upstream of the transmission, and the gear stage of the transmission is basically set in accordance with the vehicle speed even in case a vehicle is run by the power of the second electric motor/generator. Therefore, if the vehicle speed is changed when the vehicle is run by the motor, an upshifting or a downshifting of the transmission is carried out in response to the change in the vehicle speed. However, in case of such a “motor running”, a rotation of the motor is smoother than that of the engine so that noise of the motor resulting from the rotation thereof is also smaller than that of the engine. This means that the fluctuation of the drive force and the noise resulting from a speed change operation of the transmission under the “motor running” are much conspicuous in comparison with those under an “engine running”. Especially, if the “motor running” can be selected by an operation of a switch, a driver or a passenger may feel a running condition different from an intended running condition. That is, a driver or a passenger may feel uncomfortable feeling.